Error: “NetStream.Play.StreamNotFound” while playing f4m file using NetStream object

Hello,

I try to play a F4m file

var url:String = "some_http_url.f4m";

connect_nc = new NetConnection();
connect_nc.connect(null);

stream_ns = new NetStream(connect_nc);

stream_ns.client = this;
video.attachNetStream(stream_ns);   

stream_ns.addEventListener(NetStatusEvent.NET_STATUS, onNetStatusEventHandler);
stream_ns.play(url);

but I have this error "NetStream.Play.StreamNotFound"

Thanks